Mechanical fiber separation under torsional stress

Spruce wood (Picea abies) samples were pretreated at different temperatures and, in some cases, with sodium sulfite solution under normal conditions f or the chemimechanical pulping of wood chips. The pretreated samples were s imultaneously subjected to torsion and compression stresses at temperatures ranging from 20 to 90 degrees C. The load-deformation relationship was ana lyzed at each temperature. The failure zones were studied using the scannin g electron microscope technique. The results showed that the maximum torque decreased with increasing temper ature under the deformation conditions applied. Sulfonation of the samples gave a similar effect, although to a smaller extent. The elastoplasticity o f the samples, as viewed in terms of the twist angle at failure, was also a ffected by the pretreatment. While axial compression had a major effect, su lfonation only caused a marginal change. Microscopic studies of the failure zone showed that when the temperature in creased, the fracture plane traveled around fibers instead of through them, thus causing less fiber damage. Sulfonation improved fiber separation and decreased fiber damage. The separation of ray cells from the fiber tracheid s was improved, particularly by sulfonation. Increasing temperature and sul fonation changed the fracture plane from the secondary wall of fibers towar ds the more lignin enriched primary wall and middle lamella.